The present invention is directed to emulsion binders that cure to form strong thermosetting bonds, with no formaldehyde generation for use in the formation of nonwoven products. These binders can be broadly classified as general purpose binders and heat resistant binders.
General purpose non-woven products have gained acceptance in the industry for a wide range of applications, particularly as replacements for woven fabrics in constructions such as for facings or topsheets in diapers, incontinent pads, bed pads, sanitary napkins, hospital gowns, and other single and multi-use nonwoven products. For such uses it is desirable to produce a nonwoven product which closely resembles the drape, has flexibility and hand softness of a textile and yet is as strong as possible.
Heat resistant binders are used in the formation of asphalt-like roofing membranes such as those used on flat roofs. Polyester webs or mats about one meter in width are formed, saturated with binder, dried and cured to provide dimensional stability and integrity to the webs, thereby allowing them to be used on site or rolled and transported to a converting operation where one or both sides of the webs are coated with molten asphalt. The binder utilized in these webs plays a number of important roles in this regard. If the binder composition does not have adequate heat resistance, the polyester web will shrink when coated at temperatures of 150.degree.-250.degree. C. with the asphalt. A heat resistant binder is also need for application of the roofing when molten asphalt is again used to form the seams and, later, to prevent the roofing from shrinking when exposed to elevated temperatures over extended periods of time. Such shrinking would result in gaps or exposed areas at the seams where the roofing sheets are joined as well as at the perimeter of the roof.
Since the heat resistant binders used in these structures are present in substantial amounts, i.e., on the order of about 25% by weight, the physical properties thereof must be taken into account when formulating for improved heat resistance. Thus, the binder must be strong enough to withstand the elevated temperatures but must also be flexible at room temperature so that the mat may be rolled or wound without cracking or creating other weaknesses which could lead to leaks during and after impregnation with asphalt.
Conventional binders generate formaldehyde upon curing. Heat resistant binders have traditionally been prepared from acrylate or styrene/acrylate copolymers containing N-methylol functionality which generate formaldehyde. Other techniques for the products of heat resistant roofing materials include those described in U.S. Pat. No. 4,539,254 involving the lamination of a fiberglass scrim to a polyester mat thereby combining the flexibility of the polyester with the heat resistance of the fiberglass. These binders also generate formaldehyde. Conventional general purpose binders include formaldehyde-generating urethane and acrylic polymeric resins. These reins are typically the source of substantial quantities (about 200 to 500 ppm or more in the ambient air) of formaldehyde during curing. Formaldehyde has been identified as a hazardous substance and a great deal of attention has been focused in recent years on a substitute binder free of formaldehyde generation. The current limit on formaldehyde concentration in the workplace is about 3 ppm in the ambient air.
The prior art with regard to non-formaldehyde systems has suggested using binders such as urethane polymers and acrylic polymers, as disclosed in Van Norden Morin, U.S. Pat. No. 2,837,462, Baker, Jr., U.S. Pat. No. 4,207,367, Fulmer et al., U.S. Pat. No. 4,381,332 and others. These alternative systems do not appear to have achieved substantial commercial significance.
A need exists for an improved emulsion binder than can be used in heat resistant applications as well as general purpose applications without generating formaldehyde during cure. The preferred heat resistant binders will provide nonwoven fabrics having high tensile strength and heat resistance without generating formaldehyde. The preferred general purpose binder will provide a nonwoven fabric with high wet tensile strength, moisture and solvent resistance, and tear resistance wihtuot generating formaldehyde.